I don't justify an expenditure of $2500 just out of personal preference but many do. You're going to be hard pressed to find a good wide angle for $150. The 24, 28 and 35mm lenses mostly spoken about in this thread range from about $275-700. The low end of that scale is for a very well used lens. I know I've been looking to get a 24mm or 28mm f/2 for some time now.
As for rangefinder v. SLR, what you get in a wide is the potential to shoot relatively distortion free. Pardon me if you're not still talking about rangefinder cameras and lenses.
That is, a true non-retrofocal lens will only give you geometric distortion, not barrel or pincushion like you find in all but the very finest, most corrected SLR lenses. These RF lenses include the: 2.1cm Nikkors for RF and F, CZ 35mm f/2.8 Biogon, Jupiter-12, 21mm Super Angulon, 2.5cm Nikkor, Carl Zeiss Hologon 15/16mm and Topogon 25mm, 21mm and 28mm Leica Elmarits with very deep rear elements, and there are surely a few more out there.
What you sacrifice by using these is even field illumination at wide apertures but that is the same with all wide angles, only more pronounced with true non-retrofocal lenses.
Rangefinders can be a great expense but at the same time they can be had relatively inexpensively and you can get some extremely high quality optics for them for little money if you know where to look and are patient.
As for your FM, Find out what focal lengths you like shooting at and maybe divide that by 1.5 to get your next widest lens. That way you won't have too much overlap that you can otherwise make up for with your feet.
Phil Forrest